Building a 77+ mph Stage 2+ (stock ECU) RXP

I know some people have not hit 75+ mph with their Riva Stage 2 RXPs, so I thought I would throw out some ideas and personal experience.

Install the Riva Stage 2 setup, but get either the RR SC impeller or the new Riva Stage 3 SC setup, and put in Jerry’s metal washers – do not stay with ceramic! Install 42 lb injectors to handle the increased boost of the upgraded supercharger. Install the upgraded Riva aluminum intercooler tubing, the plastic deforms and leaks boost. Do not install the impeller cone on the 15/20 impeller. Strongly consider the Rotax front motor mount and a stainless steel wear ring. I am running the Riva intake grate and 2 degree wedge – it works well for me, especially with the RE blockoffs. For easier dialing in, get either the Riva or Skat adjustable venturi, with 81 – 83 mm rings. 81 – 81.5 seems to be the sweet spot, install either one and then bend the trailing edge of the 15/20 to get your rpms where you want them. Nothing wrong with 8100 rpms either, better to be a little on the low side of rpms rather than hitting 8150 all the time, you may be leaving speed on the table at 8150 rpms and not even know it – you may be hitting the rev limiter.

Reasons some people are not even hitting 75 mph with their Stage 2 setups: boost leaks, either through the BOV or the crappy plastic intercooler tubing or a leaking blockoff plate. I am convinced there are quite a few people who have minor boost leaks and don’t know it … get the upgraded intercooler tubing and be done with it. Install a boost gauge; the best place seems to be to drill and tap the IC blockoff plate. You WILL NOT hit 75 mph on a stock ECU setup without the Solas 15/20 impeller on an RXP. If you are running a tweaked stocker or a 14/19, forget it, you will not see 75+ mph. It is critical to run the 15/20 4 blade (not sure on the 3 blades on a stage 2, no data yet). Install a very good air filter setup, a 4 inch intake works best, either front or rear intake area. Wear rings – make sure yours is very good or you will not get good speeds – consider stainless.

My personal experience – the best numbers I have run were in 70 degree air, 60 degree water, ran 77.7 mph at 8100/8150 rpms. But this was with my seat completely off … my previous best with the seat on was 77.3 mph, numerous times, even in warmer weather. I am still running the Riva 3 inch air intake, so my current setup is somewhat starved for air. I will be setting up a 4 inch rear intake over the winter, some have gained as much as 100 rpm over the Riva air intake.

My ski shortcomings – I am still running the Riva air intake as mentioned above. Also running a stock waterbox at the moment, but have a modified one to use in the spring again. Not much gains from a modified waterbox from what I have seen. I have not tried the ride plate skegs yet. My intercooler tubing is just crappy 2.5” crush bent muffler pipe (ends flared to 2.75”), but at least it does not leak! I will be upgrading to the new Riva aluminum tubing over the winter.

My ski oddities – I did cut the ‘sharks teeth’ out of the tunnel. I have no idea if this affects speed at all, my thinking is it does not since others have also run 77 mph with a stage 2, so no reason to cut your sharks teeth out unless you break a few off. I also put in a layer of carbon fiber and West Marine epoxy on the water side of the pump tunnel to reinforce the fiberglass and gelcoat there. Again, likely no impact at all on speed, I just feel good knowing there will be no cracks in the gelcoat here (check yours periodically). I have done a little light sanding on the ride plate, but very little. I am running stock sponsons and RE blockoffs. Get the RE blockoffs … a huge improvement – no porpoising at any speed now.

I think it is very possible to get a stock ECU boat comfortably in the 78 mph range (in fact I think Jerry already did that with Carey’s boat). Supposedly the skegs add .7 – 1.2 mph alone. If I added the skegs, and a better air intake and pitched back a little, I would likely be at 78+ mph. Then add the Riva S3 SC and that adds even more … Possibly 79 mph? Maybe … and that is with stock ECU, 8100 rpms and no valvetrain upgrades.

Just a little food for thought … Dan (RXP in MI) has a very similar setup to mine, but was only getting 74.8 mph with a 14/19. He had a good rear air intake, but could not get to 75+ mph. He switched to a stock 15/20 and ran 76.9 mph with no other changes than the impeller … this was with the seat on, the same day I ran my best speeds. A Stage 2 setup right, with a good 15/20 turning 8100 rpms, no boost leaks and good air intake should be going 75 – 77 mph. Relatively cheap and reliable for that speed.

I know others have also hit 77 – 78 mph with this setup, so throw out any additional ideas you might have …

This is a great post! The information and experience you share here can help everyone who has that "need for speed". I, for one, will refer to this several times this winter while working on my "To Doo" list... Thanks!

Thanks guys, I think the first person to hit 79 mph with a stock ECU should get a prize ... I wish my season was longer to do some more testing.

Jarrett, that one still is absolutely illogical ... I just don't get it either. I have always thought that on the same boat, same prop, additional engine rpms should = additional speed, always. I just don't know ... but the 'Stage 2+' just seems like a bit of a sweet spot in terms of price/reliability/performance to me. I think if all of us continue to try things and share the results we might actually get a stock ECU boat close to 80 mph by next spring ... maybe.